Modular cushioning pads are often used in energy absorbing devices, such as suspension systems, draft gears and shock absorbers. In such devices, the pads are loaded in compression to absorb energy. Much attention has been directed to pads using bodies of elastomer in combination with rigid inserts and, specifically, to variations in the contours of the elastomeric bodies which will provide desirable performance characteristics. Such elastomeric cushioning pads often have central openings to facilitate stacking the pads and maintaining them in alignment by using a guide disposed through the communicating central openings. The rigid inserts enhance the load carrying ability of the cushioning pads in compression, improve the stability of the pads, and cooperate with the guide to maintain the pads in alignment. The contour of the elastomer may be varied to control more precisely the load-deflection characteristics of the pad and the bulging of the elastomer, and to enhance the fatigue life of the pad under repeated or cyclical loading.
Elastomeric cushioning pads generally have one of two general configurations. One configuration type includes an elastomeric cushion bonded between a pair of spaced rigid plates. The other type includes a single rigid plate having an elastomeric cushion bonded to one or both sides. The present invention relates to the latter type. Through proper design of the cross-sectional shape of the elastomeric cushion, a geometrical loading versus deflection characteristic can be obtained. This geometrical characteristic provides a near-linear spring rate versus load curve and results in a pad that has a near-constant natural frequency, irrespective of the load. Such performance characteristics are highly desirable in that excessive deflections can be prevented and good stability and control of motion is possible. Typical prior art pads utilizing a single rigid plate are disclosed in Willison U.S. Pat. No. 2,656,182 and Zanow U.S. Pat. No. 3,086,765.
The pad disclosed in Willison U.S. Pat. No. 2,656,182 includes a metal plate having a central opening and an elastomeric cushion bonded to one or both sides of the plate. The cushion is ring-like in shape and is disposed symmetrically relative to the plate opening. In vertical cross section, the elastomeric cushion is generally symmetrical in contour; that is, the inside and outside surfaces of the elastomeric cushion have the same contour but in opposite directions. In discussing the performance characteristics of such an elastomeric cushion, Willison specifically mentions that the contours of the inside and outside surfaces of the cushion cause the elastomer to fold or crease, during compression, along a line spaced a sufficient distance back from the peripheral edge of the cushion to protect the bond at the metal-elastomer interface. According to Willison, if the crease, during compression of the elastomer, should occur at or near the peripheral edges of the cushion, a high stress would be established at the bond interface which would result in rapid failure of the bond under repeated loadings. Applicant contends that the formation of this crease, while removing the high stress concentration from the bond interface, establishes a similar high stress area at the crease which also detrimentally affects the fatigue life of the pad under repeated loadings.
As discussed in Zanow U.S. Pat. No. 3,086,765, compression loading of a pad such as that disclosed in Willison establishes a resultant force radially outward of the pad which causes uneven flow and outward bulging of the elastomer, as compared to inward flow and bulging. Zanow employs the basic principles disclosed in Willison but adjusts the contour of the elastomer to be asymmetrical in vertical cross section so as to distribute more evenly the flow of elastomer inwardly and outwardly. The asymmetry is achieved by altering the slopes of the inside and outside surfaces of the elastomeric cushion. Creasing of the elastomer during compression loading is still employed to protect the interface bond. Zanow mentions that his asymmetrical contour results in a pad of greater capacity than previously available in symmetrical pads having a comparable volume of elastomer. It is believed that the alleged increase in capacity is a result of establishing higher stresses in the elastomer due to the opposing radial directions of bulging.